In order to prevent other birds from stealing their winter food supply, Eurasian jays, a member of the crow family, try to stealthily hide their collection when a potential thief is near. However they are also minimizing their sounds in an attempt to stay unnoticed and spy on other birds' hoards.

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Eurasian jays are creative hoarders that bury food like acorns and seeds in thousands of locations over the course of a year so they can retrieve their reserves when food is scarce. However, these hidden reserves do not always remain a secret and the stealing of these hoards is a common practice among species in the animal kingdom.

Researchers at the University of Cambridge first conducted an experiment where they gave the jays options to hide food in two different mediums: gravel and sand. The birds' preferences for using these different substrates were tested when they were alone, when they had another bird that could see and hear them and when there was another bird that could hear but could not see them.

The researchers found that if a Eurasian jay is hiding it's plunder and hears but does not see another bird nearby it will hide its cache in the less noisy substrate, in this case, the sand. Researchers conclude that this is done to avoid drawing unwanted attention from potential thieves that might then try to spy on the bird.

In the second experiment, researchers measured how many times the subjects chirped or made vocal noises depending on who was around.

They found that pilfering birds vocalize less when spying on another bird caching compared to when they are alone. The researchers believe that the jays are quieter in order to prevent their presence becoming known to the caching bird that might otherwise hide their cache elsewhere or stop hiding food.

Rachael Shaw, a PhD student at the University of Cambridge and co-author of the paper, said: "As humans, we understand that other people can hear what we are doing, but there is only limited evidence for this ability in other animals. Our study of Eurasian jays is the first to report that a member of the crow family will suppress acoustic information by vocalising less when spying on another individual that is caching."

The new research can be found in the journal Proceedings of the Royal Society B.